[Haskell-cafe] Re: Is "take" behaving correctly?

Conor McBride wrote:

Hi folks

On 12 Sep 2007, at 00:38, Brent Yorgey wrote:

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On 9/11/07, PR Stanley wrote: Hi take 1000 [1..3] still yields [1,2,3] I thought it was supposed to return an error.

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If for some reason you want a version that does return an error in that situation, you could do something like the following:

take' n _ | (n <= 0) = [] take' n [] | (n > 0) = error "take': list too short" | otherwise = [] take' n (x:xs) = x : take' (n-1) xs

I'm not sure why you'd want that, though. The standard implementation gracefully handles all inputs, and usually turns out to be what you want.

There are two sides to this form of "grace", though. I'll grant you, it's quite hard to pull off a successful fraud without a degree of aplomb.

I always hope that key invariants and hygiene conditions can be built into the static semantics of programs, but where that's impractical somehow, I prefer if the dynamic behaviour makes it as easy as possible to assign the blame for errors. In such circumstances, I'd like operations to complain about bogus input, rather than producing bogus output.

Then you want a runtime assertion checking error helpful Data.List replacement. Could you use Control.Exception.assert and make a wrapper for Data.List? http://www.haskell.org/ghc/docs/latest/html/users_guide/sec-assertions.html